During a northward interplanetary magnetic field on 27 March 2001, the Low Energy Neutral Atom (LENA) imager on the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft in the magnetosphere observed an enhanced emission in the direction of the very high‐latitude magnetopause. Simultaneous observations from IMAGE/LENA and SuperDARN radar show that the LENA emission appears concurrently with the enhancement of the sunward flow of the reverse convection in the ionosphere. The field line mapping from the magnetosphere to the ionosphere suggests that the source ions for the LENA emission are in the sunward flow region. Although the direction of the emission is relatively stable, its direction changes slightly so that the emission may shift poleward or equatorward. From these observations, we suggest that LENA can monitor the ion entry caused by cusp reconnection and that the reconnection site moves on a timescale of several minutes.

We have surveyed 3.5 years of Polar Thermal Ion Dynamics Experiment (TIDE) data between 1 January 2000 and 30 June 2003, when Polar apogee paths (sections of orbits with geocentric distances r > 5 RE) were in the dayside outer magnetosphere, to study the spatial distribution of thermal ions and the magnetospheric convection paths of the thermal ions, as a function of interplanetary magnetic field (IMF) orientation. We have found a dawn‐dusk asymmetry in the occurrence of detectable thermal ions above the instrument threshold. The occurrence rate was significantly higher at the duskside. The probability of observing thermal ions, particularly at 1300–1600 local time (LT) near the magnetopause, was >50%, compared with <30% at the dawnside. We interpret the thermal ion events as the result of plasmaspheric drainage plumes, as observed by IMAGE spacecraft or geosynchronous orbiters. The episodic appearance of the thermal ions in the outer magnetosphere could be a significant factor for the dynamo process of global magnetospheric convection. The variation of the convection pattern due to the IMF orientation is consistent with equatorward and poleward reconnection scenarios that superimpose dayside convection driven by reconnection on top of the background convection driven by a viscous interaction at the magnetopause, together with the corotation of the magnetospheric plasma with the ionosphere.

Images of charge‐exchanged neutrals from ion outflow during a period of substorm recovery are supported by auroral oval images and simultaneous in situ ion outflow measurements. From these combined remote sensing and in situ measurements, the ion outflow is shown to consist of ion conics. The energy of these conics likely increases from <10 eV at 0.7 RE to >10 eV above this altitude. These first global images of the instantaneous outflow show that outflow occurs over nearly the entire dayside auroral oval at high latitudes, including the cusp. The broad local time extent of the outflow is observed for ∼2.5 hours. Outflow on the nightside may be weaker than on the dayside and/or may have different energy‐altitude dependence.

We consider the formation of the plasma sheet and geosynchronous region (nonstorm) ring current in the framework of collisionless test particle motions in three‐dimensional magnetospheric fields obtained from self‐consistent MHD simulations. Simulation results are compared with observations of the near‐Earth plasma sheet from the Polar spacecraft during 2001 and 2002. Many particles were initiated in two regions representative of the solar wind source upstream of the bow shock and the polar wind source outside the plasmasphere, both of which are dominated by protons (H+). Proton trajectories are run until they precipitate into the atmosphere, escape from the simulation space, or become stably trapped. These calculations produce a database of proton characteristics in each 1 RE3 volume element of the magnetosphere and yield velocity distributions as well as bulk plasma properties. We report results reflecting steady growth phase conditions after 45 min of southward interplanetary field, BZ = −5 nT (BY = 0), and for conditions resulting after 2 hours of northward BZ = +5 nT. The results for simulated velocity distributions are consistent with the Polar soundings of the current sheet from lobe to lobe and with AMPTE/CCE observations of (nonstorm) ring current region protons. The simulations help us identify the differentiation between solar and polar wind H+ ions in observations. The weak NBZ ring current‐like pressure is primarily polar wind protons, while the moderately active SBZ ring current‐like pressure is primarily solar wind protons. The solar and polar wind contributions to the SBZ ring current are comparable in density, but the solar protons have a higher average energy. For SBZ, solar wind protons enter the nonstorm ring current region primarily via the dawn flank and to a lesser degree via the midnight plasma sheet. For NBZ, solar wind protons enter the ring current‐like region via the cusp and flanks. Polar wind protons enter the nonstorm ring current through the midnight plasma sheet in both cases. Solar and ionospheric plasmas thus take different transport paths to the geosynchronous (nonstorm) ring current region and may thus be expected to respond differently to substorm dynamics of the magnetotail.

A dynamic fluid semi-kinetic (DyFK) model is used to simulate the cleft ion fountain. Ion field-aligned flows are modeled for a flux tube convecting along an empirical model specified convection trajectory across the polar ionosphere from the cusp/cleft region. In this DyFK model, the collision-dominated portion of the flux tube is treated with a moment-based fluid model for altitudes from 120 to 1100km, while a generalized semi-kinetic model is used for the 800km to 3 RE altitude region. Convection-driven frictional ion heating and the effects of cusp/cleft soft electron precipitation in the F region/topside ionosphere, and centrifugal acceleration of ions and wave-driven transverse ion heating at high altitudes, are incorporated into the present simulations of the ion field-aligned transport. The modeled evolution of the O+ flow parameters at 5500km altitude exhibits typical patterns for the cleft ion fountain: O+ field-aligned flows are upward over the dayside cusp/cleft and remain upward for 5 or more degrees latitude into the polar cap, with decreasing magnitude towards the pole; the flows turn downward at about 86° invariant latitude and tend to increase in magnitude (downward) across the polar cap from dayside to nightside; the O+ density also displays apparent day–night asymmetry with higher density on the dayside. The simulated field-aligned flow pattern is in qualitative agreement with the observations from the thermal ion dynamics experiment (TIDE) during a Polar satellite southern perigee pass. The simulated ion densities and field-aligned fluxes are in general consistent with those observed. It is also shown in systematic simulations that the day–night asymmetry of the O+ density across the polar cap from dayside to nightside may be directly controlled by the cleft ion fountain, while the H+ density asymmetry is probably caused by day–night variations in solar illumination.

Analysis of ENA data from the LENA instrument on the IMAGE spacecraft shows that the terrestrial atmosphere is a copious emitter of energetic neutral atoms (<300 eV) under all conditions. When activity is low, the observed emissions are concentrated close to the Earth and are presumed to be the high‐energy tail of the warm oxygen geocorona, with energies <2 eV. When activity increases, the relative abundance of the higher‐energy neutrals increases, and the emissions can be seen farther from the Earth. Because of the close correlation between the postperigee ENA flux (fluxes seen 1–2 hours after spacecraft perigee) and Ap and the fact that the postperigee fluxes are seen when no magnetic storm is in progress we conclude that many of the emitted ENA come from the auroral zone and are produced by energized ionospheric ions rather than by precipitating energetic ions. In more spectacular events, such as the Bastille Day storm event (14–16 July 2000), oxygen neutral emissions produced by precipitation of keV ring current oxygen ions can also make an important contribution to the total neutral emission. We conclude that diurnal variation in ENA emissions is a winter hemisphere feature that is absent in the summer hemisphere. As activity increases, the altitude range of the auroral oval ENA emission region increases.

The contribution of ionospheric plasma to the Earth's magnetosphere has been recognized for more than 3 decades. The magnitude of this contribution has become more evident over that same time period with the observed magnitude of the low‐energy ionospheric supply increasing as the measurement techniques improved. Estimates based on Dynamics Explorer measurements in the mid‐1980s suggested that the ionospheric plasma supply is sufficient to populate the plasmasphere, plasma trough, plasma sheet, and magnetotail lobes. Recent measurements from the Thermal Ion Dynamics Experiment on the Polar spacecraft have been used in conjunction with an ion trajectory model to reexamine the process and magnitude of the ionospheric supply of magnetospheric plasma. These measurements reveal the energy, pitch angle, and flux characteristics of the upward flowing polar wind over broad regions of the high‐latitude ionosphere. Onboard measurement of spacecraft potential is found to be a fundamental element in interpreting the measured ion outflow. Newly derived polar wind fluxes are determined to be near 6.0 × 107 ions cm−2 s−1 at 5000 km altitude during local winter and magnetically quiet conditions. Using the measured ionospheric source characteristics in combination with the trajectory code reveals the nature of the ionospheric/magnetosphere filling process and shows that the ionospheric source is sufficient to supply the observed densities and energies of the plasma sheet and magnetotail lobes. Many of the ionospheric particles are further transformed to ring current energies and locations after circulation through the plasma sheet. This measurement/calculation approach is able to show which regions of the high‐latitude ionosphere are important for plasma sheet/ring current filling. The ionospheric sources used in the calculations include the dominant polar wind, the cleft ion fountain, and the auroral zone.

A statistical study of the ion outflow versus energy input is performed by using multi‐instrument data (TIDE, EFI, MFI, HYDRA) from Polar during its perigee auroral passes in the year 2000. Several important physical quantities connected to the ion outflow have been investigated, including the Poynting flux from the perturbation fields (below 1/6 Hz), the electron density, temperature, and the electron energy flux. The perturbation fields used here to calculate the Poynting flux may be associated with the small‐scale quasi‐static field structures of the field‐aligned currents or/and the very low frequency Alfvén waves (below 1/6 Hz), which are both proven to be important energy sources for powering the aurora. Our results show that the field‐aligned ion outflow flux correlates best with the Earth‐directed Poynting flux and the precipitating electron density and also demonstrates almost no correlation with the electron energy flux and temperature. The findings from this Polar study are similar to those from FAST. The general corroboration between the independent data sets of the two spacecraft suggests that the empirical ion outflow scaling laws can be established, which will be beneficial to global simulation efforts. Our results show that at 6000 km altitudes fi = 106.836±0.028S0.535±0.086 and fi = 106.650±0.063ne0.484±0.147, where fi is the total field‐aligned ion outflow flux in 1/cm2/s, S is the Poynting flux in ergs/cm2/s, and ne is the electron density in 1/cm3.

We investigated the nonlinear impact of the plasma sheet density on the total energy of the storm‐time ring current by means of a numerical simulation that self‐consistently solves the kinetic equation of ring current protons and the closure of the electric current between the magnetosphere and ionosphere. Results of the simulation indicate that when the convection electric field is self‐consistently coupled with the ring current, the total energy of the ring current ions trapped by the Earth's magnetic field is roughly proportional to ∼Nps1/2, where Nps is the plasma sheet density. This nonlinear response results from the strengthened shielding electric field with increasing Nps. The total energy is almost proportional to Nps when using an empirical convection electric field, which is independent of the condition of the simulated ring current. An empirical relationship between Nps and the solar wind density was used to estimate time‐dependent Nps. The result shows that the calculated Dst* tends to overshoot the observed one when the non‐self‐consistent electric field is employed. A better agreement was obtained with the self‐consistent electric field. We suggest that the nonlinear response of the ring current to Nps is one of the mechanisms that impedes the growth of the storm‐time ring current. Another mechanism is probably the saturation of the polar cap potential drop for high solar wind electric field.

We studied dynamics of O+ ions during the superstorm that occurred on 29–31 October 2003, using energetic (9–210 keV/e) ion flux data obtained by the energetic particle and ion composition (EPIC) instrument on board the Geotail satellite and neutral atom data in the energy range of 10 eV to a few keV acquired by the low‐energy neutral atom (LENA) imager on board the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) satellite. Since the low‐energy neutral atoms are created from the outflowing ionospheric ions by the charge exchange process, we could examine variations of ionospheric ion outflow with the IMAGE/LENA data. In the near‐Earth plasma sheet of XGSM ∼ −6 RE to −8.5 RE, we found that the H+ energy density showed no distinctive differences between the superstorm and quiet intervals (1–10 keV cm−3), while the O+ energy density increased from 0.05–3 keV cm−3 during the quiet intervals to ∼100 keV cm−3 during the superstorm. The O+/H+ energy density ratio reached 10–20 near the storm maximum, which is the largest ratio in the near‐Earth plasma sheet ever observed by Geotail, indicating more than 90% of O+ in the total energy density. We argued that such extreme increase of the O+/H+ energy density ratio during the October 2003 superstorm was due to mass‐dependent acceleration of ions by storm‐time substorms as well as an additional supply of O+ ions from the ionosphere to the plasma sheet. We compared the ion composition between the ring current and the near‐Earth plasma sheet reported by previous studies and found that they are rather similar. On the basis of the similarity, we estimated that the ring current had the O+/H+ energy density ratio as large as 10–20 for the October 2003 superstorm.

We present observations of Pc 1 waves (∼0.6 Hz) that occurred shortly after a strong (>20 nPa) compression of Earth's magnetosphere at 1321 UT, 18 March 2002. Intense Pc 1 waves were observed at several high‐latitude ground stations in Antarctica and Greenland from 1321 UT to beyond 1445 UT. Two wave bursts were recorded at the Polar satellite at 1338 and 1343–1344 UT as it passed outbound in the Southern Hemisphere at 1154 local time (SM magnetic latitude of −22° and near L = 7.5) in good magnetic conjunction with the Antarctic. The pressure increase created a significant population of protons between a few hundred eV and several keV, whose fluxes were mostly perpendicular to B. These protons seem to have replaced the quiescent stream of protons (presumably convected from the plasma sheet) that existed before this increase. There was also a nearly two‐order‐of‐magnitude increase in the population of thermal/suprathermal (0.32–410 eV) protons. The generation of ion cyclotron waves is expected to limit the proton temperature anisotropy A, defined as T ⊥ /T ∥ − 1. The ion cyclotron instability driven by the observed hot ion temperature anisotropy is studied using two models, with and without the presence of cold background plasma. Peaks in the calculated instability as a function of time show excellent agreement with the times of the Polar wave bursts, which were measured a few tens of seconds after maxima in the instability calculation. The time delay is consistent with the propagation time to the spacecraft from a source nearer to the equatorial plane. The hot proton population at Polar appears to be driven back to stability by a sudden increase in very field‐aligned protons having energies less than the hot perpendicular population, suggesting a different source for the two populations. These observations confirm the importance of both the energization and/or increase in population of protons transverse to B in the several keV range (possibly betatron acceleration as a result of the pressure pulse), and the presence of greatly increased fluxes of lower energy protons (100s of eV to a few keV), predominantly aligned along B, in determining whether the particle population is unstable at a given time.

We report an event observed by the Low‐Energy Neutral Atom (LENA) imager on 18 April 2001, in which enhanced neutral atom emission was observed coming from the direction of the Sun and from the general direction of the subsolar magnetopause. The enhanced neutral atom emission is shown to be primarily a result of increased solar wind charge exchange with the Earth's hydrogen exosphere, that is, enhanced neutral solar wind formation, occurring in conjunction with a southward turning of the interplanetary magnetic field (IMF) which moves the magnetopause closer to the Earth. It is shown that the neutral atom flux under compressed magnetopause conditions is extremely sensitive to changes in the IMF north‐south component.

The Low Energy Neutral Atom (LENA) imager on the IMAGE spacecraft in the dayside magnetosphere can detect neutral particles that are emitted in the magnetosheath flow. During a period of dynamic pressure of 4–6 nPa and interplanetary magnetic field (IMF) Bz of −5 to 3 nT on 12 April 2001, LENA on IMAGE at (XGSM, YGSM, ZGSM) ∼ (4 RE, 0 RE, 6 RE) observed significant emission in the direction of the high‐latitude magnetosheath. Detailed analyses have revealed that the high‐latitude sheath emission consists of two parts: the stable emission at the higher latitudes and the lower‐latitude emission that occurs on and off. During the interval of this event, the Polar spacecraft was located at somewhat lower latitudes than IMAGE in similar noon meridian, and the plasma observations with the Thermal Ions Dynamic Experiment showed that the entry of the cusp ions happens in concurrence with the appearance of the lower‐latitude LENA emission. This coincidence strongly suggests that the cusp ions flowing earthward charge exchange with the hydrogen exosphere. For the higher‐latitude emission, its stability suggests that the source is associated with the structure persistently existing, which is consistent with the recent result showing that the sheath flow in the cusp indentation can create neutral atom emissions. Comparison of the LENA emission and ACE solar wind suggests that the lower‐latitude LENA emission occurs during the southward tilting of dawnward IMF, indicating that this emission is associated with the earthward ion flux along the newly reconnected field lines. Hence this unique event for the simultaneous observations strongly suggests that LENA monitors the entry of the ions in the cusp, which is triggered by the southward tilting of the IMF, and that the significant flux of the cusp ion entry occurs equatorward of and separately from the cusp indentation.

Observations of strong solar wind proton flux correlations with ROSAT X-ray rates along with high spectral resolution Chandra observations of X-rays from the dark Moon show that soft X-ray emission mirrors the behavior of the solar wind. In this paper, based on an analysis of an X-ray event observed by XMM-Newton resulting from charge exchange of high charge state solar wind ions and contemporaneous neutral solar wind data, we argue that X-ray observations may be able to provide reliable advance warning, perhaps by as much as half a day, of dramatic increases in solar wind flux at Earth. Like neutral atom imaging, this provides the capability to monitor the solar wind remotely rather than in situ.

Using the Comprehensive Ring Current Model (CRCM), which self‐consistently solves the kinetic equation of ring current protons and the closure of the electric current between the magnetosphere and ionosphere, we have studied how different changes in the ionospheric conductivity affect the strength of the ring current. The conductivity for F10.7 = 250 × 104 Janskys (Jy) (solar maximum condition) results in a ring current that is about 29% stronger than for F10.7 = 70 × 104 Jy (solar minimum condition). The conductivity at equinox results in a ring current that is about 5% stronger than at solstice because the two‐hemisphere height‐integrated conductivities at equinox are higher than at solstice. This would be a new mechanism for explaining the semiannual variation of Dst. Simulation with a realistic auroral conductivity estimated from the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE)/Far Ultraviolet Imager (FUV) auroral imager data reveals the fact that auroral brightenings do not significantly change the intensity of the ring current. The overshielding condition is found to be produced when the auroral conductivity decreases abruptly near the Dst minimum, triggering a rapid decay of the ring current. The ring current is shown to be influenced not only by the interplanetary magnetic field and the solar wind but also by solar radiation and morphological features of the auroral electron precipitation as well.

The existence of a secondary stream of neutral atoms inside the heliosphere arriving from about 285degrees ecliptic longitude, which is about 30degrees higher than the nominal upstream direction of the inflowing interstellar gas, has been proposed recently based on a wide variety of observations from many different missions. We will discuss the LENA/IMAGE measurements in detail and conclude that the secondary stream is composed mainly of hydrogen atoms at an energy of about 1 keV. We will discuss some possible explanations for the origin of the secondary stream, with the most likely source being the region upstream of the termination shock.

Four different data sets pertaining to the neutral atom environment at 1 AU are presented and discussed. These data sets include neutral solar wind and interstellar neutral atom data from IMAGE/LENA, energetic hydrogen atom data from SOHO/HSTOF and plasma wave data from the magnetometer on ISEE-3. Surprisingly, these data sets are centered between 262° and 292° ecliptic longitude, ∼10–40° from the upstream interstellar neutral (ISN) flow direction at 254° resulting from the motion of the Sun relative to the local interstellar cloud (LIC). Some possible explanations for this offset, none of which is completely satisfactory, are discussed.

In this paper, we present in-situ observations of processes occurring at the magnetopause and vicinity, including surface waves, oscillatory magnetospheric field lines, and flux transfer events, and coordinated observations at geosynchronous orbit by the GOES spacecraft, and on the ground by CANOPUS and 210° Magnetic Meridian (210MM) magnetometer arrays. On 7 February 2002, during a high-speed solar wind stream, the Polar spacecraft was skimming the magnetopause in a post-noon meridian plane for ~3h. During this interval, it made two short excursions and a few partial crossings into the magnetosheath and observed quasi-periodic cold ion bursts in the region adjacent to the magnetopause current layer. The multiple magnetopause crossings, as well as the velocity of the cold ion bursts, indicate that the magnetopause was oscillating with an ~6-min period. Simultaneous observations of Pc5 waves at geosynchronous orbit by the GOES spacecraft and on the ground by the CANOPUS magnetometer array reveal that these magnetospheric pulsations were forced oscillations of magnetic field lines directly driven by the magnetopause oscillations. The magnetospheric pulsations occurred only in a limited longitudinal region in the post-noon dayside sector, and were not a global phenomenon, as one would expect for global field line resonance. Thus, the magnetopause oscillations at the source were also limited to a localized region spanning ~4h in local time. These observations suggest that it is unlikely that the Kelvin-Helmholz instability and/or fluctuations in the solar wind dynamic pressure were the direct driving mechanisms for the observed boundary oscillations. Instead, the likely mechanism for the localized boundary oscillations was pulsed reconnection at the magnetopause occurring along the X-line extending over the same 4-h region. The Pc5 band pressure fluctuations commonly seen in high-speed solar wind streams may modulate the reconnection rate as an indirect cause of the observed Pc5 pulsations. During the same interval, two flux transfer events were also observed in the magnetosphere near the oscillating magnetopause. Their ground signatures were identified in the CANOPUS data. The time delays of the FTE signatures from the Polar spacecraft to the ground stations enable us to estimate that the longitudinal extent of the reconnection X-line at the magnetopause was ~43° or ~5.2 RE. The coordinated in-situ and ground-based observations suggest that FTEs are produced by transient reconnection taking place along a single extended X-line at the magnetopause, as suggested in the models by Scholer (1988) and Southwood et al. (1988). The observations from this study suggest that the reconnection occurred in two different forms simultaneously in the same general region at the dayside magnetopause: 1) continuous reconnection with a pulsed reconnection rate, and 2) transient reconnection as flux transfer events. Key words. Magnetospheric physics (Magnetopause, cusp and boundary layers; Magnetosphere-ionosphere interactions; MHD waves and instabilities)

Results are presented from a survey of cold ion observations in the near‐Earth magnetotail using data from the Polar Thermal Ion Dynamics Experiment (TIDE). During the interval from July to December of 2001, Polar had its apogee (∼9.5 R E ) near the equatorial plane in the tail region of the magnetosphere. It is shown that a lobal wind is ubiquitous in the inner tail, with low‐energy (<300 eV) ions streaming from the ionosphere downtail. These lobal winds often pass through the plasma sheet, forming bidirectional streams, in addition to the unidirectional beams seen at higher magnetic latitudes. The observance of bidirectional streams is inversely, although weakly, correlated with geomagnetic activity. Bidirectional streams are interpreted as indicating the minimum size of the closed flux tube region. The reduced frequency of bidirectional streams with activity level times is consistent with the thinning of the plasma sheet during these times. It is inferred from the universality of these observations during Polar's passage through the inner tail region that the ionosphere is a continuous supplier of plasma to the near‐Earth magnetosphere. The high occurrence rate of these streams means that during geomagnetic disturbances, it is not necessary to wait for outflow and magnetospheric circulation in order to supply the inner magnetosphere with ionospheric ions; these cold streams are an immediately available supply of ionospheric‐origin particles.